Most cameras come with a small amount of internal memory that can store just a few pictures. Then, it's up to the buyer to get a new memory card with a proper amount of storage. Your camera choice may change depending on what your needs are as far as storage goes. Some cameras have storage systems designed to keep thousands of pictures at a time, and some cameras just focus on quickly accessing a smaller amount of pictures.
When you find a potential camera, you need to do your research on it before you make the purchase. Some cameras accept several different formats, but most just have one type of card that will work with them. Find out what this is. Also, check to see if it has an upper limit.
Some cameras will only recognize 1 GB of space, and if you buy a 2 GB memory card then you will have wasted your money. You can find all of these details on web pages for the product that list all of its specifications. If those are unclear, then you can ask other people who have bought the same product.
Now that you know what kind of card to get, you can decide on the amount of space you need. With memory cards being so cheap these days, it is tempting to simply buy the largest card available, and never worry about running out of space. But, if you feel that there is no way you could fill up all that space, then you might as well stick with something smaller. The average high resolution picture takes up between 2 and 16 MB. Take the maximum number of pictures you would want to take, and multiply it by that, then round it up to the next memory card option available.
So maybe you decide you want a 2 GB SD card, or a 512 MB MMC card. Once you've decided, you can find sellers all over the internet. Usually if you are patient and watch for sales at retail sites, you can find memory cards for extremely cheap. You may also be able to find a camera bundle deal in which you buy the camera and it comes with the necessary memory card. Either way, you should definitely find a memory card if you plan on using your digital camera a lot.